feat(ADR-261 M2): multi-bit + large-N ANN scaling study — measured, no crossover (refutes M1 prediction) (#1066)
* feat(ADR-261): multi-bit (b∈{1,2,4}) quantized HNSW traversal + scaling harness
Generalize the SymphonyQG-style quantized-traversal HNSW from 1-bit Hamming to a
b-bit-per-dimension code (b ∈ {1,2,4}), mirroring ADR-156 §10's multi-bit RaBitQ
scheme (rotate via FHT Pass-2, uniform mid-rise scalar quantizer over [-3,3],
ranked by per-dim L1). b=1 is byte-for-byte the original construction (codes in
{0,1} ⇒ L1 == Hamming), pinned by one_bit_build_bits_matches_legacy_build.
Bytes/node scales linearly: 128-d → 16/32/64 B for b=1/2/4.
- hnsw_quantized.rs: QuantizedHnswIndex::build_bits(...,bits,...), bits()/
bytes_per_node() accessors, code-L1 greedy+beam traversal. build(...) kept as
the b=1 backward-compatible entry point. +4 tests (multi-bit recall regression,
bits clamp, bytes/node, legacy parity).
- ann_measure.rs: build_indices_bits / build_quant_bits / run_scaling_study +
best_float_op / best_quant_op; scaling_report (#[ignore], --release) and a
CI-safe scaling_study_small_is_consistent.
- ann_bench.rs: 2-bit and 4-bit quant criterion benches over the shared graph.
ruvector lib 151 → 156 passed, 0 failed, 1 ignored (scaling_report).
Co-Authored-By: claude-flow <ruv@ruv.net>
* docs(adr-261): record M2 multi-bit scaling study — measured, no crossover (refutes M1 prediction)
Multi-bit (b∈{1,2,4}) quantized HNSW traversal + N∈{10k,100k,250k} scaling study,
measured on this box. No crossover at any (N,b): at 10k more bits help (ratio
0.19→0.48×, b≥2 reaches 0.90 recall) but quant stays slower than float HNSW at
equal recall; at 100k/250k quant recall collapses (b=4: 1.0→0.788→0.624, never
≥0.90) while float holds ≥0.92. The predicted large-N crossover moved the wrong
way. Published negative with the mechanism explained. ADR-261 §11.
Co-Authored-By: claude-flow <ruv@ruv.net>
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Co-authored-by: ruv <ruvnet@gmail.com>
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